It could also be Co-op. That segment has been in development longer. They had running versions of it months ago. But I do hope its Eldar, or both. Edit: Especially since Nathan is Tweeting this: https://twitter.com/ennstamatic/status/736186494050816000
eldar definitely need more than 2 week of works, animation in particular need a lot of extra work, so it's probally going to be pve (that was planned to be on founder server months ago).
They also said that PvE will need to be on the Founder's Server for a long time (even months), before it is released to the Early Access Servers. So if they want to keep up to that, then they will need to release PvE rather soon.
Space Marine Driver: "Damn Hawks! They are shitting all over my PREDATOR! How dare they!" Pooping Hawk "Baaawk! Baaawk!" (the translator couldn't translate this) Space Marine Sergeant: "This is not poop! This is BIRD DROPPINGS!" Space Marine Driver: "What do you mean by that?!? Shit IS shit!" Space Marine Sergeant: "No! This bird is dropping GRENADES!" Space Marine Driver: "What a terrible pun, Sir..." BOOOOOM!!!!! (sound of Predator exploding) Pooping Hawk <incapacitated> Predator, 800xp Pooping Hawk <destroyed> Predator, 500xp Pooping Hawk <grenade> Space Marine Driver, 100xp Pooping Hawk <grenade> Space Marine Sergeant, 100xp Doublekill! 50xp